1. Field of the Invention
The present invention relates to a relief valve assembly, that can prevent abrupt pressure drops during relief operations in an oil pump, and thus can perform superior relief operations.
2. Description of the Related Art
When operating vehicle engines at high rotation speeds, engine lubricating oil is supplied by an oil pump. Most oil pumps are equipped with relief valves for pressure control. When the supplied oil in the oil pump is under abnormally high pressure, the relief valve assembly diverts the oil to a different route to hold the oil pressure constant, so as to avoid damage of the machinery.
However, if the high pressure is decreased at once with such a relief operation, the supplied amount of oil is not constant, and pulsations or bubbles occur, and as a result, oscillations occur, which may have a harmful effect on the machinery to which the oil is supplied, Therefore, attempts have been made to increase the initial amount of the return oil from the relief valve little by little, in order to avoid abrupt pressure drops in the relief valve assembly.
A specific example of such a relief valve assembly is disclosed in JP H5-94220A, which is provided with a relief valve for pressure control. In this relief valve assembly, one end is closed, and at the other end an oblong first adjustment hole 76 and second adjustment hole 78 are formed in communication with one another along the axial direction of the relief valve at a lateral position in a relief valve unit of perforated cylindrical shape having a bottom.
In JP H5-94220A, a pressurized fluid, such as oil, flows into the relief valve unit of cylindrical shape having a bottom, and when, depending on the pressure acting on the bottom portion, the relief valve unit moves against the spring force of a holding spring, then abrupt pressure drops do not occur when the pressurized fluid, such as oil is discharged, because the pressure adjustment hole provided near the bottom portion of the relief valve is formed such that its aperture area changes in accordance with the movement of the relief valve.
The pressurized fluid, such as oil, that has entered the relief valve unit of cylindrical shape having a bottom is first discharged through the aperture opening in the oblong first adjustment hole 76 of the pressure adjustment holes in the relief valve unit. However, the aperture area is increased little by little due to the surface area change of the pressure adjustment holes, namely the oblong first adjustment hole 76 provided with a rectangular cross section and the circular second adjustment hole 78 whose aperture area is larger, and an aperture to the pressure side chamber of the relief valve unit develops abruptly from the rectangular cross section of the aperture of the first adjustment hole 76 with the smaller aperture area, and it is very difficult to attain more stable and improved start-up properties,
Furthermore, if no pressurized fluid, such as oil, is given into the relief valve unit of cylindrical shape having a bottom, then the structure is such that it is not possible to form an aperture with the pressure adjustment hole of the relief valve unit, so that when there is foreign matter in the pressurized fluid, such as oil, the foreign matter tends to accumulate at the bottom portion and at the pressure adjustment hole near the bottom portion of the relief valve unit, and there is the danger that foreign matter enters between the sliding faces of the relief valve unit and the valve housing, thus hindering or blocking the sliding of the relief valve unit.
Due to protruding of the valve from the valve path into the valve housing when the relief valve unit slides, the sliding support portion of the valve unit is gradually reduced, so that there is the danger that the movement of the relief valve unit becomes unstable. This can result in a gradual tilting when the valve unit moves.
Therefore, every time the valve unit moves along the valve path during the relief operations the edge of the valve unit erodes the surface of the valve path, causing damage, so that the reaction of the valve unit to abnormal pressures may deteriorate. Also, the machining of the first adjustment hole 76 and the second adjustment hole 78 in the lateral circumferential wall of the valve unit is not easy, and it is difficult to reduce costs,
As a result of many of hours of thorough research trying to solve these problems, the inventors have succeeded in developing a relief valve assembly with a very simple structure, with which a relief operation with superior reaction is realized, thus solving the above-mentioned problems. This relief valve assembly comprises a relief valve, a valve housing including a valve path along which the relief valve can slide, a relief inflow portion formed on one end of the valve housing, and relief discharge portions formed at a substantially intermediate location of the valve path of the valve housing and provided with small aperture portions and large aperture portions formed in communication with one another, wherein a side where the relief discharge portion is provided with the small aperture portions is the side near the relief inflow portions